Study of the dual role mechanism of water-soluble additive in low temperature thermally-induced phase separation

In the NIPS method, polyethylene glycol-400 (PEG400) was a common water-soluble additive and usually played the role of a pore-forming agent due to NIPS double-diffusion. In this paper, the new role of non-solvent PEG400 in the low temperature thermally-induced phase separation (L-TIPS) was analyzed. PEG400 were used to regulate the phase separation mechanism of the dope solution, and then to improve the membrane structure and performance. The membrane structure was regulated by gradually increasing the PEG400 content, the role evolution of water-soluble non-solvent PEG400 on the phase transformation mechanism and the effect on tensile strength of the polyvinylidene fluoride (PVDF) membrane were studied. The results show that the increase of PEG400 content can increase the cloud point temperature of dope solution, accelerate the thermally induced phase separation (TIPS) effect and inhibit the negative effect of the non-solvent induced phase separation (NIPS) effect on membrane structure. Meanwhile, the increase of PEG400 content can shift the TIPS effect of the dope solution system from solid-liquid phase separation to liquid-liquid phase separation and inhibit the growth of PVDF spherulites during the TIPS process. The dual role mechanism of PEG400 together eliminates the NIPS finger-like voids and the TIPS sphere-packed aggregation structure, which have the negative impact on tensile strength of the PVDF membrane. The PVDF membrane shows excellent mechanical properties; the tensile strength of the membrane reaches 3.12 MPa (more than 3-6 times higher than the membranes reported), pure water flux reaches 391 L m−2 h−1, and the rejection rate of carbon ink is 100%.